Distillates derived from a Fischer-Tropsch process or from the hydroprocessing of vegetable oils can be composed of normal or straight chain paraffins (n-paraffins) in the C8 to C30 range that have relatively high melting points. While these distillates can have excellent cetane numbers, in some cases, however, they also can have poor cold flow properties. For example, such long chain paraffins can crystallize into waxy solids under cold temperatures, which result in the poor flow properties. Cold flow properties of a hydrocarbon stream are often characterized by measuring cloud point, pour point, and cold filter plugging point (CFPP). Such distillates as described above can have high cloud point values of at least about 4.4° C. (40° F.), high pour point values of at least about 4.4° C. (40° F.), and high CFPP values of at least about 4.4° C. (40° F.). In order to improve these properties, the hydrocarbon stream can be subjected to hydroisomerization where the n-paraffins are converted to branched paraffins (iso-paraffins), which have better cold flow properties.
Current hydroisomerization techniques typically employ a three-phase system (gas/liquid/solid catalyst), such as conventional trickle bed technology, to convert n-paraffins into iso-paraffins. In these systems, the continuous phase throughout the reactor is a gas phase, and large amounts of hydrogen gas are generally required to maintain this continuous gas phase throughout the reactors. However, supplying such large supplies of gaseous hydrogen at the operating conditions needed for isomerization adds complexity and expense to the system.
For example, in order to supply and maintain the needed amounts of hydrogen in a continuous gas phase system, the resulting effluent from the hydroisomerization reactor is commonly separated into a gaseous component containing hydrogen and a liquid component. The gaseous component often is directed to a compressor and then recycled back to the reactor inlet to help supply the large amounts of hydrogen gas needed to maintain the continuous gaseous phase therein. Conventional distillate hydroisomerization units typically operate at about 3.45 MPa (500 psig) to about 8.27 MPa (1,200 psig) and, therefore, require the use of a recycle gas compressor in order to provide the recycled hydrogen at the high pressures of the reactor. Often such hydrogen recycle is from about 1,200 to about 5,000 SCF/B, and processing such quantities of hydrogen through a high-pressure compressor adds complexity and cost to the hydroisomerization unit.
On the other hand, while such three-phase systems generally require large amounts of hydrogen to maintain the continuous gas phase, the hydroisomerization reactions typically do not consume significant amounts of hydrogen. While hydrogen is generally needed to effect isomerization, this reaction generally does not consume hydrogen. Some hydrogen may be consumed, however, because a small amount of cracking may also occur in isomerization reaction zones in which hydrogen is consumed. As a result, there tends to be a large excess of hydrogen throughout the isomerization system when conducted in a continuous gas phase that generally is not needed for the isomerization reactions. Such excess hydrogen is typically separated from the resulting effluent streams prior to further processing, which requires additional separation zones and vessels. As discussed above, if this excess hydrogen is recycled to the hydroisomerization inlet to help supply gas to the system, the hydrogen must still be processed through high-pressure compressors in order to supply the hydrogen at the needed high pressures of the reaction vessel. As a result, not only does conventional three-phase hydroisomerization require costly, high-pressure compressors, these systems have an excess of hydrogen that is generally not consumed in the process.
Two-phase hydroprocessing (i.e., a liquid hydrocarbon stream and solid catalyst) also has been proposed in some cases to convert certain hydrocarbonaceous streams into other more valuable hydrocarbon streams. For example, the reduction of sulfur in certain hydrocarbon streams may employ a two-phase reactor with pre-saturation of hydrogen rather than using a traditional three-phase system. See, e.g., Schmitz, C. et al., “Deep Desulfurization of Diesel Oil: Kinetic Studies and Process-Improvement by the Use of a Two-Phase Reactor with Pre-Saturator,” Chem. Eng. Sci., 59:2821-2829 (2004). These two-phase systems only use enough hydrogen to saturate the liquid-phase in the reactor. As a result, the reaction systems of Schmitz et al. do not provide for decreasing hydrogen levels due to hydrogen consumption during the reaction process, thus the reaction rate in such systems decreases due to the depletion of the dissolved hydrogen. Hydrodesulfurization is a process that requires large amount of hydrogen and has a large hydrogen consumption to effect the desired sulfur reductions.
Other uses of liquid-phase reactors have been to hydrocrack and hydrotreat hydrocarbonaceous streams. However, hydrotreating and hydrocracking also typically require large amounts of hydrogen to effect their conversions; therefore, a large hydrogen demand is still required even if these reactions are completed in liquid-phase systems. As a result, to maintain such a liquid-phase hydrotreating or hydrocracking reaction and still provide the needed levels of hydrogen, prior liquid-phase systems require the introduction of additional diluents or solvents into the feed to dilute the reactive components of the feed relative to the amount of dissolved hydrogen. As a result, the diluents and solvents provide a larger concentration of dissolved hydrogen relative to the feed to insure adequate conversion rates can occur in the liquid-phase. Larger, more complex, and more expensive liquid-phase reactors are needed in these systems to achieve the desired conversions.
Although a wide variety of process flow schemes, operating conditions and catalysts have been used in commercial petroleum hydrocarbon conversion processes, there is always a demand for new methods and flow schemes that provide more useful products and improved product characteristics. In many cases, even minor variations in process flows or operating conditions can have significant effects on both quality and product selection. There generally is a need to balance economic considerations, such as capital expenditures and operational utility costs, with the desired quality of the produced products.